1. Field of the Invention
This invention relates generally to circuit breakers having a magnetic trip assembly in which the magnetic field induced by an abnormal current unlatches a latchable operating mechanism to trip the breaker, and more particularly, to such a magnetic trip assembly which employs a solenoid to translate the magnetic field into mechanical movement to cause actuation of trip mechanism.
2. Background Information
Circuit breakers provide protection for electrical systems from electrical fault conditions such as current overloads and short circuits. A common type of circuit breaker used to interrupt abnormal conditions in an electrical system incorporates a thermal trip device which responds to persistent low levels of overcurrent and a magnetic trip assembly which responds to higher levels of overcurrent in a fraction of a second. An example of such a circuit breaker is disclosed in U.S. Pat. No. 4,528,531. In such circuit breakers, the thermal trip device comprises a bimetal which bends in response to a persistent low level overcurrent passing through it to unlatch a latchable operating mechanism. The latchable operating mechanism is spring operated to open electrical contacts which interrupt the current. In a large number of commercial circuit breakers a rotatable trip bar is provided to initiate the unlatching of the latchable operating mechanism in response to either the electrothermal stimulus or an electromagnetic stimulus. The electrothermal stimulus is related to I.sup.2 t=K; or stated another way, the amount of overload current present over a predetermined period of time. The electromagnetic stimulus is related to short circuit conditions, sometimes referred to as an instantaneous tripping situation. Generally, the calibration of the electrothermal stimulus is related the angular swing through which the trip bar rotates in response to impingement thereon by the bimetallic member. On the other hand, response to the short circuit condition is related to how quickly an armature can be attracted to an electromagnetic member. In each case, the current flowing in the main terminals of the circuit breaker provides input into the electrothermal or electromagnetic response. As the size of the circuit breaker apparatus is reduced during miniaturization, the need for a highly calibrated, rugged and repeatable electromagnetic trip mechanism exists, that can reduce manufacturing costs.